1. Field of the Invention
This invention relates to pulley assemblies and, more particularly, to a pulley assembly, such as one that would be used on an auxiliary apparatus on an engine, that is driven by a power transmission belt.
2. Background Art
In a typical automobile engine construction, multiple auxiliary apparatus are incorporated, each with a shaft that is rotated to effect operation thereof. Typical of such auxiliary apparatus are alternators, air compressors, water pumps, etc. Each apparatus has an associated pulley, as does a drive shaft on the engine, about which an endless belt is trained in a serpentine manner. The engine drive shaft develops an operating torque that is transmitted through the belt to the various auxiliary apparatus through the pulleys thereon.
Since the output shaft on the engine undergoes rotational speed changes, due to a number of factors, such as during expansion strokes, the resulting speed of the belt travel likewise changes. These speed changes may cause slippage between the belt and the various pulleys on the auxiliary apparatus. The speed fluctuations may also cause significant variations in the belt tension.
Both the slip and speed variation conditions have adverse consequences. One problem is that there may be generation of excessive noise during running. These conditions may also shorten the life of the belt and/or the auxiliary apparatus. Other problems may also result.
The above problems are particularly troublesome with an alternator. Alternators have shafts that typically have a high moment of inertia. This may aggravate both the slip and tension variation conditions. Still further, variations in speed of the output shaft translate into corresponding speed variations in the alternator shaft. This may cause a failure of the alternator or reduce its useful life. The result is potentially a detrimental effect on power generation efficiency.
The above problems have been known in the industry. Attempts have been made to devise structures that would avoid these conditions and associated problems. In Japanese Patent Laid-Open Publication No. 2006-177548, a pulley assembly is disclosed with a rim having a cylindrical surface against which a belt is run, a center hub installed inside the rim, a rubber elastic ring disposed between the rim and the center hub, and a rotation restricting unit. The rotation restricting unit consists of a central plate that is square in shape and fixed to an outer circumferential surface of the center hub. Elastic contact parts are disposed on an inner circumferential surface of the rim at regular intervals. As the rim rotates relative to the center hub, the rubber ring is twisted so that slippage or variation in tension of the belt can be limited. Relative rotation between the parts is also limited by contact between the elastic parts and the central plate when a predetermined degree of rotation occurs between these components.
If the rubber ring is twisted repeatedly within the permitted range of relative movement of the parts, the rubber ring may be prone to failure within a relatively short time period.
In Japanese Patent Laid-Open Publication No. Hei 6-200956 a pulley assembly is disclosed with an outer ring against which a belt is wrapped, an inner ring inside the outer ring, and a spring body formed from an elastomer material and disposed between the outer ring and the inner ring. The spring body is attached to one of the inner ring and outer ring so as to be fixed against relative rotation with respect thereto. The spring body is slidably connected through a sliding layer to the other ring. When the inner ring and outer ring rotate relative to each other, the spring is twisted, thereby to prevent slippage or variations in tension. Additionally, a frictional force is generated between the sliding layer and ring, to prevent excessive twisting of the spring body.
Even though the sliding layer is provided between the spring body and ring, it is still possible that the spring body could be extensively twisted when there is erratic, and large magnitude, change in engine speed. This increases the likelihood that the spring body will break, potentially within a relatively short time period.